1. Field of the Invention
The present invention relates to a substrate for a flat panel display and an electroluminecence element using the substrate.
2. Related Background Art
Electroluminecence elements are practically used as a liquid crystal display (LCD) and a back light for a clock.
Electroluminecence elements are elements using an emitting-light phenomenon of materials when a voltage is applied, that is, an electroluminecence (EL) phenomenon.
There are two types of electroluminecence elements using a fluorescent body made of an inorganic material; one is a dispersed-type electroluminecence element having a structure where electrode layers are provided on the top and bottom of a light emission layer formed by an organic substance or vitreous enamel having levigated fluorescent material dispersed therein, and the other is a type of thin film electroluminecence element that has a laminated body having a structure where a thin film light emission body is sandwiched by a pair of thin film insulators and further by a pair of electrode layers, provided on an electrically-insulating substrate.
Furthermore, each of the types has a direct-current voltage driving type and an alternating voltage driving type. The dispersed-type electroluminecence element has been known long before, and has an advantage of being easy to produce, but the use thereof is limited because the illuminance is low and the life time is short. On the other hand, the thin film electroluminecence element has been widely used in recent years because it has characteristics of high illuminance and long lifetime.
FIG. 2 is an oblique, perspective view showing schematically the structure of a typical double-insulating-type thin film EL element as a conventional EL element. This thin film EL element 10 has a structure where a lower electrode layer 3, a lower insulator layer 4, a light emission layer 5, an upper insulator layer 6, and an upper electrode layer 7 are overlaid in that order on an electrically-insulating substrate 2.
The substrate 2 is transparent, and constituted by a blue glass plate used for a liquid crystal display, a PDP, etc. The lower electrode layer 3 is formed of ITO (Indium Tin Oxide) having a film thickness of about 0.2 to 1 μm. The lower insulator layer 4 and the upper insulator layer 6 are respectively a thin film having a thickness of about 0.1 to 1 μm and formed by sputtering or vapor deposition, and are made of Y2O3, Ta2O5, Ai3N4, BaTiO3, or the like.
The light emission layer 5 has a film thickness of about 0.2 to 1 μm. The upper electrode layer 7 is made of metal such as Al The lower electrode layer 3 and the upper electrode layer 7 are so patterned like stripes that one of them is a row electrode and the other is a column electrode, the stripes being arranged orthogonal to each other. The intersections of the row electrode and the column electrode form picture elements, and by selectively applying an alternating voltage or a pulse voltage to a specific picture element via this matrix pair of the electrodes, the light emission body is made to emit light, and the emitted light is produced from the substrate 2 side.
The lower insulator layer 4 and the upper insulator layer 6 of this thin film EL element 10 have a function to limit an electric current flowing through the light emission layer 5, thereby preventing the dielectric breakdown of the thin film EL element and functioning to achieve a stable light emission characteristic. Because of this function, the thin film EL element 10 having this structure is widely in practical use commercially as well.
As the fluorescent material forming the light emission layer 5, ZnS in which Mn emitting yellowish orange light is doped has been mainly used from the point of view of easiness in forming a film and light emission characteristics. In order to make a color display, it is inevitable to adopt light emission materials that emit three primary colors of red, green, and blue.
Known as these materials are SrS with Ce doped or ZnS with Tm doped for emitting blue light, ZnS with Sm doped or CaS with Eu doped for emitting red light, and ZnS with Tb doped or CaS with Ce doped for emitting green light.
Disclosed in a literature, Shosaku Tanaka, “The technology trend of recent displays”, Monthly Magazine Display, April 1998, pp. 1-10 are ZnS, Mn/CdSSe, etc., as materials for red light emission; ZnS:TbOF, ZnS:Tb, etc., as materials for green light emission; and SrS:Cr, (SrS:Ce/ZnS)n, CaGa2S4:Ce, SrGa2S4:Ce, etc., as materials for blue light emission. Furthermore, SrS:Ce/ZnS:Mn, etc., are disclosed as materials for white light emission.
In addition, SrS:Ce from among the above materials being used as the blue light emission layer of the thin film EL element 10 is disclosed in a literature, X. Wu, “Multicolor Thin-Film Ceramic Hybrid EL Displays”, IDW (International Display Workshop), 1997, pp. 593-596. It is also disclosed in this literature that forming a light emission layer of SrS:Ce in the H2S atmosphere by an electron beam vapor deposition method can produce a light emission layer with high purity.
However, there is still a problem in terms of structure with this thin film EL element 10, which has to be solved. That is, when displays having a large area are produced, the problem is that the light emission layer 5 may break down due to a local decrease of the insulation-withstand voltage at the steps at the pattern's edge of the lower electrode layer 3 in the lower insulator layer 4, which is a thin film, or defects in the lower insulator layer 4 caused by dust, etc., generated in the production process, which defects are difficult to eliminate.
Because the problem is fatal to display devices, there has been a bigger obstacle to the thin film EL element being widely used as a display having a large area compared with a liquid crystal display or a plasma display.
Disclosed in Japanese Patent Publication No. 7-44072 is an EL element using a electrically-insulating ceramic substrate as the substrate 2 and a thick film dielectric layer instead of the thin film insulator layer as the lower insulator layer 4 in order to solve the problem that defects are easy to occur in the insulator layer, which is a thin film. The EL element disclosed in the literature is different in structure from the conventional thin film EL element in that, in order to produce light emitted by the light emission layer 5 from the opposite side from the substrate 2, the upper electrode layer 7 is a transparent electrode layer.
The thick film dielectric layer is formed to have a thickness of several tens to several hundreds μm, which is several hundreds to several thousands times that of the thin film insulator layer. Therefore, the number of the occurrences of dielectric breakdown due to the steps formed by the lower electrode layer 3 or pinholes formed by dust, etc., in the production process is reduced greatly, and thus it has an advantage that high reliability and high yield in the production are obtained.
Note that although using the thick film dielectric layer poses a problem that an effective voltage applied to the light emission layer 5 decreases, this problem is overcome in for example Japanese Patent Publication No. 7-44072 by forming the thick film dielectric layer of a compound-perovskite high dielectric material including lead.
As described above, by using the thick film dielectric layer having a high dielectric constant, the problem can be solved that breakdown in the light emission layer may be caused by a local decrease of the insulation-withstand voltage at the steps at the pattern edge or defects in the insulator layer caused by dust, etc., occurring in the production process, which problem is likely to occur in the case of the thin film insulator layer.
However, in forming a thick film dielectric layer, it is necessary to coat or deposit a dielectric green over a substrate by use of a screen print, a green sheet method, or the like and then sinter it at a high temperature of 700 to 800° C. or above. Accordingly, a material forming the substrate needs to have an enough mechanical heat resistance at that temperature and a coefficient of thermal expansion that coincides with that of the formed thick film dielectric layer, and needs to be low in reactivity with the thick film dielectric material. Therefore, in the prior art, an alumina ceramic substrate has been mainly used as the substrate.